1. Field of the Invention
The present invention relates to an image-forming apparatus such as a variable magnification copying apparatus. More specifically, the present invention is directed to such an image-forming apparatus which is provided with an illumination mechanism for making the surface potential on the non-image forming area substantially equal to the surface potential on the bright portion of the image forming area of the photosensitive medium. For the purpose of this specification, "non-image forming area" means such area of the photosensitive medium which is not used to form an image thereon as a result of the change of image magnification. By the term "the bright portion of image forming area" is meant the portion of image forming area of the photosensitive medium which corresponds to the bright portion of the projected light image.
2. Description of the Prior Art
At present there are known and used various types of image-forming apparatus a representative example of which is a copying apparatus. In recent years variable magnification copying apparatus also have been proposed which are provided with the function to change the image magnification of the original image formed on the photosensitive medium.
In the known copying apparatus provided with variable magnification facility, when the image magnification is .times.1 (one-to-one magnification) or more (copy making with enlargement), the whole surface of the photosensitive medium is usually used fully as the image forming area. However, when the image magnification is less than .times.1 (copy making with magnification), a portion of the photosensitive medium remains unused as a non-image forming area. Since the non-image forming area is not subjected to the exposure of light corresponding to the image of the original, there is formed on the area a latent image equivalent to the dark portion of the original image. Therefore, at the step of developing a latent image formed by an image-wise exposure with charged colored developing particles such as toner to obtain a positive image of the original image, the latent image on the non-image forming area is also developed together with the development of the desired latent image on the image forming area. In other words, a large amount of toner is uselessly consumed to develop the non-image forming area. As a large amount of toner is consumed, the ratio of toner to carrier liquid or powder is rapidly reduced, and the degradation of the developing agent is accelerated even when the toner on the unnecessarily developed part is recovered for reuse. Furthermore, such a large amount of toner consumed by the non-image forming apparatus has an adverse effect on the cleaning device of the apparatus. As well-known to those skilled in the art, most image-forming apparatus of the type mentioned above are provided with a cleaning device to clean off the residual toner after transfer from the surface of the photosensitive medium. When the amount of toner used by the photosensitive medium is large, the wear of the cleaning device becomes larger.
A solution to the above problem hitherto widely used is to illuminate the non-image forming area by an auxiliary light source other than the light source for illuminating the original. To prevent the non-image forming area from being undesirably developed with toner, the surface potential on the non-image forming area is made to decay or erase by illuminating the area by an auxiliary light source. At the developing step, therefore, toner does not adhere to the non-image forming area. The transfer material can be separated from the photosensitive medium without being made dirty by toner. The problem of too rapid consumption of toner and degradation of the developer can be eliminated by this solution.
However, the above known solution involves some difficulties in designing the auxiliary light source for illuminating the non-image forming area. In a variable magnification copying apparatus, the size of the non-image forming area in a photosensitive medium is not constant but it is variable with the change of image magnification. The auxiliary light source is, therefore, required to be able to cope with the change in size of the non-image forming area.
In the apparatus disclosed in Japanese Patent Publication No. 24,862/1979, a plural number of illumination light sources are provided near the photosensitive drum. These light sources are individually switched on and off to illuminate the non-image forming area of the photosensitive drum. The number of the light sources to be lighted is decreased or increased suitably according to the size of the non-image forming area. A disadvantage of this known method is that it needs a plural number of light sources to illuminate the non-image forming area. Another disadvantage is found in that when the magnification is changed continuously, an optimum illumination can not always be u assured for the edge portion of the non-image forming area.
In another known apparatus as disclosed in U.S. Pat. No. 3,967,896, the non-image forming area is illuminated by a beam of light from a cylindrical lamp provided in the vicinity of the photosensitive drum. A movable shutter is provided near the cylindrical lamp to adjustably limit the area of the photosensitive medium to be illuminated by the beam of light.
Both of the above-mentioned prior art apparatuses have the following common difficulties:
Since the light source for illuminating the non-image forming area is located near the photosensitive medium and the illumination light is guided directly to the photosensitive medium from the light source, it is very difficult to illuminate exclusively the non-image forming area while defining a sharp boundary between image forming area and non-image forming area.
Secondly, since the space within which the light source is provided is near the photosensitive medium, the position in which the light source is provided is limited by it. It is required to mount the light source in such position as not to interfere with the optical path for projecting the original image on the photosensitive medium. Further, the location of the light source should not interfere with other members used for carrying out the image-forming process. Of course, these limitations makes it difficult to design the apparatus in a compact form.
Lastly, the light source is required to have sufficient length to cover the largest size of the non-image forming area. Because of it, the size of the light source part becomes inevitably large.
The above problems arise not only in copying apparatus but also in other image-forming apparatus in which the size of non-image forming area of the photosensitive medium is variable.